The present invention relates to an electrostatic display element for use in a display board of an electrostatic display apparatus, and more particularly to a support mechanism or expedient of a film-like movable electrode devised so as to be electrostatically attracted and repelled by a pair of fixed electrodes.
Electrostatic display elements operating based on the principle of electrostatic force acting on a movable electrode are disclosed, for example, in a U.S. Pat. No. 4,468,663 and a U.S. patent application Ser. No. 701,859. Their constitution and principle are very similar to those of the present invention. However, the purposes to which the above inventions are directed differ essentially from the objects of the present invention, so they are not mentioned further in this specification.
As is shown perspectively in FIG. 8 and cross-sectionally in FIG. 9, which reveals the cross-section taken along a line B--B in FIG. 8, a typical example of conventional electrostatic display elements is constituted fundamentally of a pair of fixed electrodes 1 and 2 facing each other and film-like movable electrode 3 positioned therebetween. The fixed electrodes 1 and 2 have their middle portions curved upwardly to form respective semicircular inwardly projecting protrusions 12 and 22. The fixed electrodes 1 and 2 are further coated with differently colored electrically insulating layer 15 and 25 on their inside surfaces confronting each other. The above insulating layers 15 and 25, which are not shown in the perspective view (FIG. 8) for the simplicity of drawing, are differently colored. The movable electrode 3 is made of a flexible thin case film 31 plated on both surfaces with thin metallic layers 32 and 33, which make the movable electrode 3 mirror-faced. The metallic layers 32 and 33 are also omitted in the perspective view (FIG. 8) for the simplicity of drawing. The movable electrode 3 has its lower part associated with a lead plate 6 sticked thereto with an electrically conductive adhesive 7. The lower part of the movable electrode 3, including the electric lead plate 6, is kept between the fixed electrodes 1 and 2 at their lower flat partions 11 and 21, with spacers 41 and 42 interposed. The assembly is fastened by means of screw bolts 71 and 72. In the clearance made between the spacers 41 and 42, an electrically conductive paste 7 is deposited on the lower end of movable electrode 3 for the purpose of providing an electric connection between both the metal-plated surfaces 32 and 33 of the movable electrode 3. Further, the fixed electrodes 1, 2 and the electric lead plate 6 are shaped, at their lowermost parts, so as to form electric-terminal projections 14, 24 and 60.
In such a constitution of the electrostatic display element, the fixed electrode 1 and 2 are kept voltage-supplied therebetween, while the movable electrode 3 is switched selectively to either of the fixed electrodes 1 and 2. Switched to the fixed electrode 2, as is schematically shown in FIG. 10, the movable electrode 3 is attracted by and to the fixed electrode 1 (and repelled by and from the fixed electrode 2) so as to mask the insulating layer 15 of the fixed electrode 1. If the switching is selected to the side of the fixed electrode 1, the movable electrode 3 comes, as is illustrated in FIG. 11, to mask the insulating layer 25 on the fixed electrode 2. Thus, the selected switching of the movable electrode 3 changes the appearance of the display element by making the movable electrode 3 mask or expose either of the colored insulating layers 15 and 25. Incidentaly, when one insulating layer (15 or 25) is masked, not only the other layer (25 or 15) is exposed but also its image is reflected by the confronting mirror-forming surface of the movable electrode 3.
To ensure a stable operation of the display element, however, such a conventional assembly as exemplified in FIGS. 8 and 9 has two important disadvantages as described in the following.
One of the disadvantages is due to the fact that an electrically conductive "adhesive" is used to provide a good electric connection between the movable electrode and the electric lead plate 6. The use of an adhesive is liable, as is easily conceived, to cause the film-like movable electrode 3 to be wrinkled longitudinally because of the same being thin and soft. The movable electrode 3, if wrinkled even slightly, has its flexibility deteriorated largely and comes to be awkward in responding to the electrostatic forces acting thereon. The other of the disadvantages is related to the electrically conductive "paste" 7 used for making electric connection between both the metallic layers 32 and 33 at the lower end of the movable electrode 3. If an electric connection is not made between the metallic layers 32 and 33 as in case of the movable electrode 3 shown in FIGS. 10 and 11, a rapid turning of the movable electrode 3 toward the fixed electrode 1 from the fixed electrode 2 can not be assured when the movable electrode 3 is switched to the fixed electrode 2 from the fixed electrode 1, because a smooth electric discharge is put under restraint owing to a double insulator construction by a core film 31 of the the movable electrode 3 and an insulating layer 25 on the fixed electrode 2, with the metallic layer 33 kept isolated. In this sense, the electric connection by means of the electrically conductive paste 7 is unreliable, firstly because the area of contact between the paste 7 and the metallic layers 32, 33 is very small and therefore, the contact has a peril of being broken when the paste is solidified, and secondly because the electric conduction of the paste is carried by the conductive particles suspended in a non-conductive medium and therefore, a high conductivity can not be expected when the paste is solidified.